杏吧原创

What causes a star鈥檚 spin and its planets鈥� orbits to align 鈥� or not? (Image: ESO/L. Cal莽ada/Nick Risinger)

Magnetic harnesses may keep planets in line with their stars.

Some exoplanets orbit at weird angles 鈥� instead of circling in the same direction in which their star spins, their paths are tilted, and they sometimes even orbit 鈥渂ackwards鈥�. Oddly, these misalignments only seem to happen to stars more than 1.2 times the sun鈥檚 mass.

The mismatch seems to defy our understanding of how planets are born: if planets coalesce out of a disc that spins out like a pizza around a baby star, their orbits and the star鈥檚 spin should match up.

Previous explanations for the mismatch include planets having close encounters with each other or other stars; interstellar gas falling onto a new solar system and warping the planet-forming disc; or the disc itself pushing and pulling its star into a weird angle.

Now Christopher Spalding of the California Institute of Technology argues a star鈥檚 size might be the key to the puzzle.

Size matters

Smaller stars have stronger magnetic fields than big stars. As a star鈥檚 solar system forms, its magnetic field grabs charged particles in the planet-forming disc and tries to pull the star back into the disc鈥檚 plane.

鈥淚f you have a star misaligned with the disc, the magnetic field will tend to try and align the two,鈥� Spalding says.

Spalding calculated how long it would take the magnetic fields of misaligned big and small stars to grab hold of the disc and right themselves. Stars smaller than 1.2 times the sun could do it in about a million years, he found, while bigger stars could take a hundred times as long 鈥� longer than the discs stick around.

鈥淟ow mass stars have a field which can strongly impact their orientation within the disc lifetime. High mass stars have a field which takes way too long to realign them,鈥� he says.

John Papaloizou at the University of Cambridge thinks this explanation is plausible, but that the complex physics involved make the problem hard to fully solve. 鈥淲hat the authors suggest is not unreasonable,鈥� he says. 鈥淗owever, details are very difficult to work out.鈥�

Journal reference: arxiv.org/abs/1508.02365